A system for correction for composite triple beat impairments in a television signal has been described in co-pending U.S. application Ser. No. 08/059387 filed May 11, 1993, Ward et al. The rationale behind cancelling the composite triple beat (CTB) impairment is based on the concept that over a small region of any line of picture a CTB may be approximated as a constant luminance signal. Thus the effect of a CTB impairment on a small segment of an image may be modeled as a change in DC luminance by an unknown constant which if it can be discovered and filtered out should reduce the CTB. In this technique, the digitalized image is divided into vertical stripes or subpictures and the average intensity of the signal in each line of the subpicture is determined. These average intensities are then filtered by a sliding average window, a length of approximately three to five pixels. The filtered value of the average intensity is then assumed to be the correct DC level of the corresponding line of the subpicture. The noise reduction may also be further reduced by applying a multi-frame, CTB removal scheme wherein the average of the DC level is in a given subpicture is based on the average of the same subpicture in a number of consecutive frames with a sequence.
The above method with a minor modification is also capable of measuring the carrier to noise ratio of a CTB impaired picture within reasonable accuracy and to do so without being intrusive of the picture being transmitted or received.
The computational requirements of the above described system are quite high making it difficult to provide a practical system that may be used in real time to correct the TV image.
To reduce thermal (snow) noise inter frame averaging has been used. Normally to be effective e.g. to achieve a 10 decibel reduction, 10 frames are required. However, for practical purposes this poses a difficulty in that the equipment required to buffer 10 frames adds significantly to the cost.
It will be apparent that in order to reduce significantly CTB, composite second order beat (CSO) and thermal noise requires significant computation on hardware including a number of buffers etc. It has not found practical application in competitive domestic cable television systems.
U.S. Pat. No. 5,099,329 dated Mar. 24, 1992 assigned to Graphic Communication Tech of Japan describes a technique wherein the grey scale value of each pixel of a current frame is replaced with a corresponding one of the previous frame unless the pixel is part of a moving object. To determine whether or not a pixel is part of a moving object, the absolute difference between the values for pixel in the two frames is compared with a preset threshold and if greater the pixel is considered to be part of a moving object.
European patent application 404,237 filed on Dec. 27, 1990 by Phillips describes a noise suppression method for digital signals obtained by sampling an analogue TV signal and obtaining the difference between two successive sample values. The obtained difference is compared with a reference value and when the obtained difference is less the referenced, the second sample value is replaced by the preceding value.
In yet another U.S. Pat. No 4,807,034 filed Feb. 21, 1989, issued to Toshiba employs a noise reduction circuit that causes a subtracting circuit to subtract a delayed video signal (supplied from the field memory with one field time delay) from the video signal and allows the resultant signal to be output as the field difference signal. A noise component included in the video signal is extracted by a noise extracting circuit in accordance with the field difference signal and is output to the subtracting circuit. A comparing circuit compares the noise component with a pre-determined reference value to determine whether the video signal represents a motion or motionless picture and if it is a motionless picture, the memory control circuit uses the video signal supplied from the field memory. If motion is detected then the then current value is used.
The principals of the above three patents are all basically the same in that all rely on replacing the value of each pixel in a digitized picture with the value of a corresponding pixel. The main difference between the various systems is in the location of the corresponding pixel, the value of which is to be used. In U.S. Pat. Nos. 5,099,329 and 4,807,034, the location of the corresponding pixel is in the previous frame, for application Ser. No. 404,237 the corresponding pixel is in the adjacent spacial location of the same frame. All use essentially the same technique to determine motion or to detect edges. These systems reduce thermal noise only and have little if any effect on CTB or composite second order beat (CSO) impairments.